1. Field of the Invention
The present invention relates to a focus detection device for detecting the condition of focusing on a predetermined focal plane.
2. Description of the Prior Art
Generally speaking, one of the important factors required of a focus detection device for a single lens reflex camera is an applicability for a variety of interchangeable objective lens each having an individual F-number and an individual focal length. It is also an important factor that the focus detection can be carried out irrespective of the intensity of an ambient light and/or the brightness of an object.
There is proposed a focus detection system in which an auxiliary illuminating means is installed in an electronic flash device to be mounted on the camera for projecting light beams to an object when the circumference is too dark to carry out the focus control (See U.S. Ser. No. 807,642).
In such an auxiliary illuminating means, a LED (Light Emitting Diode) capable of emitting light of a wavelength of about 700 nm is used as a light source thereof, since such a light being close to the near infrared zone is less stimulative to the eye.
Although the stimulation to the eye is reduced as the wavelength of the light to be emitted becomes longer in such a focus detection system, the chromatic aberration of the objective lens is inevitably increased, and therefore, this lowers the accuracy in the focus detection. Errors in focus detection are caused due to various color temperatures in the case of the so-called passive auto focusing system (passive AF) wherein the focus control is carried out depending only on the ambient light without projecting light onto an object, since the incident light may have a various color temperatures. In the case of the so-called active auto focusing system (active AF) wherein an illuminating light is projected from the camera onto an object and the focus detection is carried out using the light reflected from the object, such bad influences due to the chromatic aberration is not so serious, when compared with the passive AF, since the color temperature of the projection light is constant.
For instance, when an interchangeable lens having a long focal length is used in the passive AF, the focal position obtained in the case when an object is illuminated by a light source having a low color temperature such as a tungsten lamp is differed by several tens .mu.m, due to the difference in the color temperature, from that obtained in the case when an object is illuminated by a light source having a high color temperature such as day-light.
In order to solve this problem, there has been proposed a focus detection system which comprises a measuring means for measuring the color temperature of a light source, means for giving parameters representing the characteristics of the chromatic aberration of the individual interchangeable lens, whereby errors in the focus detection are corrected according to those calculated due to information such as the color temperature and said parameters given (See JP-A No. 86504/1983, JP-A No. 59413/1983).
According to this system, such a problem as mentioned above is solved. However, it is difficult in an actual sense to correct errors in the focus detection exactly, since it is considerably difficult to obtain parameters representing the exact characteristics of the chromatic aberration of each interchangeable objective lens. The reason why it is difficult is that the characteristics of chromatic aberration are changed due the variety of dispersion characteristics of light about a glass material used for forming each lens element.